The invention relates to a method of measuring offset between doubly printed elements, such as halftone dots, in printed matter, in printing systems in which a test pattern is printed and then evaluated.
The quality with which halftone dots are printed by printing cylinders on paper in printing presses depends on many factors, whose origins are to be found mainly in the details of the technical design of the press (more particularly an offset litho press) and in the dynamics of press operation. For instance rotary vibrations of the cylinders, unfavorable settings between the blanket cylinder and the impression cylinder, unfavorable climatic conditions and the like, may lead to double element printing, that is to say to a double or ghost printing of halftone dots so that in addition to the densely printed halftone dot there is further adjacent shadow-like dot which is less dense and is frequently smaller in diameter. Although the printing of halftone dots is the main consideration here, it is obvious that the invention may be centered on the double printing of other features.
In order to detect the causative factors such dot doubling phenomena are observed by using test patterns. The degree of dot doubling is then expressed as the center distance between the original and ghost dot in mm. This has so far been done under a microscope using a suitable scale. The values then produced are then processed, for instance in a computer, which carries out a fourier analysis. This process is however relatively complex and furthermore the accuracy of the method depends on the observer, who measures the degree of double dot offset.